Organic electronic devices are widely used in a variety of applications. These are electrical devices such as light emitting diodes, transistors, and photovoltaic cells, that include the use of organic materials as one or more of the device components (e.g., dielectric layers, electrode layers, etc.). Organic electroluminescent devices (ELDs) such as organic light emitting diodes (OLEDs) have in recent years become commercially important. Organic materials are desirable for their light weight and low cost. Unfortunately, many organic materials suffer from low stability and low durability as compared with metallic materials.
Organic electronic devices are typically constructed with two electrodes. In the case of ELDs, an electroluminescent material is in electrical contact with both electrodes, and forms a conduction path between the electrodes. One electrode functions as a electron-injection layer, while the other electrode functions as a hole-injection layer. In some arrangements of the component layers of ELDs, a dielectric layer is present. For example, a dielectric layer may be present between all or a portion of the electrodes. Dielectric layers are also important component layers of other electronic devices such as transistors and capacitors.
An important aspect in the construction and operation of organic electric devices (OEDs) is the process of encapsulation, whereby the various component layers are protected from environmental hazards such as moisture and oxygen. For example, in the case of OLEDs, a physical barrier may be needed to protect the OLED component layers (e.g., organic and cathode materials). A common method for preparing such a barrier involves physically mating a top glass (or other suitable material) layer over, but usually not touching, the OLED device with an epoxy border. The glass, together with its epoxy border, provide the necessary environmental protection required for long-lived OLED usage. This method has a number of limitations, however, including oxygen/moisture permeability issues with the epoxy border, difficulty of manufacture, and inflexibility of the glass top layer.
In recent years, attempts have been made to develop cheaper, more rapid and more effective methods for encapsulating OEDs, particularly OLEDs. In one method, known as “direct thin-film” encapsulation, alternating and repeating layers of an organic material and a barrier layer are used. Typical organic materials are acrylate or the like, while typical barrier layer comprise a sputtered metal, metal-oxide or a dielectric layer.
One of the problems of the direct thin-film encapsulation method occurs when the barrier layer contains point defects (i.e. pin holes) in its surface. Such defects severely reduce the usefulness of the barrier layer, as they increase the amount of harmful contaminants able to cross the barrier layer. One solution to this problem is to increase the thickness of the barrier layer to eliminate defects that extend entirely through the barrier. Unfortunately, thicker barrier layers increase the weight and cost of the devices, and reduce the transparency and flexibility of the encapsulation.
Furthermore, direct thin-film encapsulation has additional drawbacks for certain types of ELDs. For example, in some OLEDs, such as those described in U.S. Pat. Nos. 6,800,722 and 6,593,687, the cathode, dielectric, and anode layers are deposited on a substrate to form the OLED stack. Cavities extending partially or completely through each of the layers are created, and a light emitting polymer (LEP) layer is deposited over the OLED stack. Within the cavities, the LEP makes contact with the anode and cathode layers. Light emission occurs in the cavity regions as electrons and holes flow through the LEP and between the anode and cathode layers. In such devices, the LEP layer forms the outermost (i.e., furthest from the substrate) layer of the OLED stack. Accordingly, the LEP layer is exposed to, and potentially damaged by, encapsulation methods that involve depositing barrier layers by chemical or physical deposition methods. For example, the LEP layer can be damaged by reactive species or solvents when the encapsulating layers are deposited by metal sputtering, chemical vapor deposition, or solution deposition methods.
A further problem suffered by layered electronic devices is breakdown or leakage of the dielectric layer, which results in unwanted current flow between the electrodes or from one electrode to ground. Commonly, such breakdown or leakage occurs because of defects in the dielectric layer. Defects include cracks and pinholes that may form when the dielectric layer is deposited and/or cured. Traditional methods for overcoming such difficulties typically involve increasing the thickness of the dielectric layer. This approach is particularly undesirable for applications in which thinner dielectric layers are needed.
There remains a need in the art to overcome the abovementioned drawbacks, as well as generally to develop new methods and materials for effectively manufacturing and protecting OEDs such as ELDs. Ideal encapsulation methods and materials would utilize materials that are readily available or easily prepared, minimize the number of process steps, and/or provide highly reproducible results and effective barrier layers without damaging the OED components. Similarly, ideal materials and methods for forming dielectric layers would utilize readily available materials and would be capable of producing thin dielectric layers having minimal defects.